Method and apparatus for enriching compounds of low water solubility from aqueous suspensions of substantially inorganic solid substances

ABSTRACT

The invention relates to a method and apparatus for enriching compounds almost insoluble in water, particularly chlorinated hydrocarbons, and thereby removing them from aqueous suspensions of solid particles contaminated by such compounds. The aqueous suspension is brought in contact with an organic hydrocarbon containing solvent, and the suspension is extracted with the organic solvent in one or more mixing stages with subsequent phase separation. Each stage comprises one or more mixing sequences in series arranged upstream of a phase separation and from where a substantial part of the organic solvent is recirculated to the first mixing sequence within that stage. The invention secures nearly complete leaching of compounds present in even less than 10% of other solid particles in an aqueous suspension.

This invention relates to a method and apparatus for enriching compoundsalmost insoluble in water, particularly chlorinated hydrocarbons, andthereby removing them from aqueous suspensions of solid particlescontaminated by such compounds.

Graphite is commonly used for anodes in metal production by electrolysisof melted metal chlorides, and in some processes additional carbon isintroduced into the melt. The chlorine evolved at the anode will formsmall amounts of chlorinated hydrocarbons leaving the cells as vapor inthe chlorine gas. Thus, when chlorine produced by the electro-smeltingproduction of magnesium metal is cooled, dust will precipitate which inaddition to sublimated magnesium chloride and other metal compounds alsocontains chloriated hydrocarbons. Although the chlorinated hydrocarbonsare only minor components of the solids precipitated from the gas, thecompounds, because of their toxicity, must be disposed of in a way thatdoes not pollute the environment.

Dust formed in the gas leaving the electrolytic cells is collected fromducts and bag filters and dispersed in water to obtain a slurry that canbe pumped.

The dust contains primarily fully chlorinated aromatic hydrocarbons:hexachlorobenzene (HCB), octachlorostyrene (OCS),pentachlorobenzonitrile (PCBN). The composition of the collected dustmay be: MgCl₂, MgO, MgOHCl, FeCl₃, HCB, OCS, PCPy, DCBF, PCBN andfragments of ceramic brick linings.

Normally the content of chlorinated hydrocarbons will amount toapproximately one part per thousand of the collected dust and thus beonly a minor constituent of the suspended solids in the slurry.

As mentioned above, the chlorinated hydrocarbons must be destructed toavoid environmental pollution, normally through combustion at elevatedtemperatures. Since the accumulated slurry contains some 70% water, thebalance being mainly noncombustible salts and solids, such hightemperature combustion, usually performed at sea, is a rather expensiveand unsatisfactory solution to the problem.

As the chlorinated hydrocarbons are almost insoluble in water, theyprobably appear as solid crystals (sublimates) embedded in the inorganiccompounds forming the major part of insolubles of the aqueous slurry.This, of course, makes it even more difficult to develop a process forcollecting and enriching the chlorinated hydrocarbons for subsequenteasy and inexpensive destruction.

From U.S. Pat. No. 3,931,001 and Japan Kokai No. 74,131953 are knownmethods for the removal of chlorinated hydrocarbons from waste water byliquid/liquid extraction utilizing conventional extraction equipmentwith a counter current packed column. It is a main object of the presentinvention to provide an extraction method which may be utilized toremove compounds almost insoluble in water, especially chlorinatedhydrocarbons, from solid particles in aqueous suspensions, where thesecompounds constitute less than 10% by weight of the total solid contentof the suspension.

Normally one would expect a liquid/liquid extraction process to beunsuitable because of the tendency of solid particles to form sludge atthe phase boundary between the liquids, and thereby to retard phaseseparation. Further, to obtain reasonable concentrations of chlorinatedhydrocarbons in the organic solvent (1-10%), the volumetric feed ratioof organic to aqueous flow to the extractor should be very low(1:100-1:1000). Also, as the solubilities in water are very low, theleaching of the chlorinated hydrocarbons will be a slow process.

According to the present invention, however, these problems can be keptunder control in a process of simultaneously leaching and extractingwhen using one or more consecutive mixers followed by a settler forphase separation. The aqueous suspension is brought in contact with anorganic solvent, the suspension is extracted with the organic solvent inone or more mixing stages with subsequent phase separation, and eachstage comprises one or more mixing sequences in series arranged upstreamof a phase separation where the two liquid phases are separated and fromwhere a substantial part of the organic solvent is recirculated to thefirst mixing sequence within that stage.

To avoid the problems caused by unfavorable phase ratios and phaseboundary sludge formation, a major part of the organic flow containingthe overflow sludge is recirculated to the mixer. Furthermore, thesuspension is acidified prior to extraction, thereby dissolving metalhydroxides and other hydrolysis products which otherwise wouldaccumulate at the phase boundary. Surprisingly it has been found that inthis way it is possible to produce a concentrate free from solids thatwithout further problems can be disposed of through simple combustion.

Further objects of and important features of the process andcorresponding apparatus will be apparent from the enclosed claims.Nevertheless, the invention will also be further elucidated in referenceto the examples given below and the simplified flow sheet in FIG. 1.

An aqueous suspension of electrosmelting dust is fed through a pipeline1 to the mixing tank 20 equipped with a stirrer 21. Hydrochloric acid isadded through line 2 in an amount sufficient to lower the pH to 3,preferably to 1, measured in the tank. The acidified mixture is then fedthrough line 7 to the first extraction stage, comprising mixers arrangedin series, here three: 22, 23 and 24, equipped with stirrers, 25, 26 and27. The organic solvent fed to the first mixer 22, will mainly berecirculated solvent from the clarifying tank 30 of the same firstextraction stage, mixed with the organic overflow from the clarifyingtank 36 of the next stage. Aqueous and solvent flows thus arecountercurrent between extraction stages. By using three mixers inseries the suspended particles will get a more uniform residence time inthe mixers. This is of great importance for the leaching yield for thesolid chlorinated hydrocarbons. From the last mixer of this stage, themixture enters the settler 28, equipped with a small stirrer 29 near theorganic overflow. The purpose of this stirrer is to break up the sludgeand thereby avoid blocking the overflow. The organic solvent overflowcontaining approximately 2% of chlorinated hydrocarbons, is then led tothe clarifying tank 30. The main part of the solvent, containing all thesludge contained in the overflow from the settler 28, is withdrawn fromthe bottom of the tank, and after mixing with overflow solvent from theclarifying tank 36 of the following stage, is recirculated to the firstmixer 22. The overflow from clarifying tank 30 is led to a productstorage tank 31 for subsequent combustion.

The underflow of settler 28 will be the aqueous suspension, nowcontaining substantially less chlorinated compounds. The suspensionflows via pipeline 11 into the second extraction stage with mixer 32 andagitator 33, and is mixed with solvent from the clarifying tank 36 ofthat stage. In mixer 32 entrained solvent in the suspension also will beexchanged, so that, when the suspension leaves the bottom of thesubsequent settler 34, substantially all chlorinated hydrocarbons wil beremoved. As in the first extraction stage, the second stage may ofcourse also contain more than one mixer 32.

The organic solvent overflows from settler 34 into the clarifying tank36 into which also fresh solvent is fed by a metering pump 38 from astorage tank 37. The overflow from clarifying tank 36 is led to thefirst mixer 22 of the first extraction stage, while thesludge-containing solvent underflow is recirculated to mixer 32 of thesecond stage.

The treated suspension underflow leaves settler 34 through pipeline 8and can normally be disposed of without further treatment.

The equipment for conducting the process thus comprises twocountercurrent stages. The process can, however, also be conducted in aone-stage apparatus comprising two or more mixers with agitators inseries and one subsequent settler, where the organic and aqueous phasesare separated. By internal recirculation of the organic phase from thesettler to the first mixer of the stage, the flow ratio of the twophases through each stage can be kept at an value optimal for extractionefficiency and phase separation, independent of the flow ratio betweenthe stages.

During phase separation in the settlers, a voluminous layer of sludge isquickly built up in the organic layer. By means of mild agitation by theagitators 29, 35 in the upper part of the settlers, however, theoverflow of organic phase into the clarifyers 30, 36 proceeds smoothly.As sludge and organic solvents are recirculated via the pipelines 10, 5,the amount of sludge in the settlers 28, 34 appears to stabilize withinreasonable time at a stationary value.

To obtain a stable operation of the settler, fluctuations in thedensity, i.e. in the solids content, of the aqueous underflow must beavoided. To secure this, the bottom of the settler is given a slopetowards the underflow outlet as indicated in FIG. 1, the inclinationexceeding the angle of repose of settling solids.

The organic solvent will preferably be an aromatic petroleum fraction(kerosene) having a flashpoint above 25° C., preferably above 60° C.

The ratio of organic and aqueous phases in the mixing sequences isgreater than 1:2, preferably greater than 1:1, rendering the organicsolvent the continuous phase, thereby improving phase separation aftereach mixing stage, i.e. within each extraction stage.

EXAMPLES

Extraction runs were performed in a continuous laboratory scaleextraction unit having one stage consisting of either one or threemixers in series, followed by one settler and a clarifier.

EXAMPLE 1: (One Mixer, one Settler)

Suspensions containing either 10% or 20% solid dust from electrolyticmagnesium smelting dispersed in water were used. The suspension wasacidified by hydrochloric acid to pH values from 0 to 2.5. The solventused was a kerosene fraction rich in aromatic compounds and a flashpoint of 66° C. The solubilities of HCB and OCS at 22° C. were found tobe 8.5% and 10% (by weight), respectively.

The equipment was in continuous operation for several days undercomplete recirculation of the solvent from the clarifier to the mixer,thereby accumulating the chlorinated hydrocarbons in the solvent. Themixer contained 2 liters of liquid. The settling area of the settler was33 cm² and the volume 1 liter. As it was found to have but minorimportance for the extraction yield, the agitator speed was later keptat a constant value sufficient to secure a satisfactory dispersion ofthe phases. The feed ratio of solvent and suspension to the mixer wasvaried between 0.8:1 and 2:1, while the total feed rate was keptconstant at 2 l/h corresponding to a mean residence time of 1 hour inthe mixer. The test temperature was the same as the ambient, 20°-22° C.

Care was taken to maintain the phase boundary level halfway down thesettler. During operation sludge built up quickly in the solvent phase.The sludge build-up in the organic phase as well as organic entrainmentin the aqueous slurry proved to be highly dependent on the solidscontent in the suspension, as well as on its pH and on the feed ratio tothe mixer of organic to aqueous phase. Later tests therefore wereperformed with a slurry made up of 10% solids in water, a pH=0.5 and avolumetric feed ratio of solvent to suspension of 1.5:1.

Even under these condition there was a considerable sludge build-up inthe solvent phase, but it appeared that by recirculating the solvent,the amount of sludge approached a stationary state condition withoutfurther build-up. By applying a mild agitation in the solvent phase, thevoluminous sludge collapsed and was easily separated in the adjacentclarifier to a clear organic product and a recirculation solventcontaminated with small amounts of aqueous sludge.

Under these conditions the organic entrainment in the aqueous slurryphase leaving the settler was measured to 0.05-0.01%.

Analytical tests of aqueous suspension entering and leaving theextraction stage gave an extraction yield of approximately 75% for OCSand 90% for HCB and other chlorinated hydrocarbons. As the chlorinatedhydrocarbons accumulated in the circulating solvent, the measuredefficiency decreased, corresponding to the content of dissolvedchlorinated hydrocarbons in the solvent entrainment. Thus a 200-foldenrichment from suspension to solvent and an organic entrainment of0.03% will reduce the overall yield by 6%.

EXAMPLE 2: (Three Mixers in Series)

This extraction run was performed under similar condition as given inExample 1, using the same settler. The single mixer of Example 1,however, was replaced by three mixers in series, each having a liquidvolume of 0.67 liter. The total mean residence time in the mixers of 1hour in Example 1 was thus still maintained. Analytical tests of theaqueous suspension now indicated that the extraction yields for OCS andHCB had increased to 87.5 and 99% respectively. Organic entrainment inthe suspension appeared to be unchanged from Example 1. Thus theaccumulation of chlorinated hydrocarbons in the solvent caused similarreduction in overall yields.

The examples show that the described procedure makes it possible toremove the chlorinated hydrocarbons from an aqueous suspension of solidparticles and to enrich these to a level suitable for recovery ordestruction, and that the sludge formation can be controlled to make asatisfactory phase separation possible. The examples show further thatthe extraction yield can be considerably improved by applying two ormore mixers in series upstream of the settler where the phase separationtakes place. Finally, the examples show that entrainment of solvent inthe treated suspension will reduce the overall yield, and therefore if ahigh enrichment and a high degree of removal is wanted, it will benecessary to use two or more countercurrent stages.

The examples describe only extraction of chlorinated hydrocarbons froman aqueous suspension of dust from an electrolytic smelter. Theprocedure according to the invention can, however, also be applied forother compounds of low water solubility contained in aqueous suspensionsof larger amounts of other insolubles.

As the compounds are practically insoluble in water, it is reasonable toexpect that they exist as solid crystals embedded in the larger amountsof other solid inorganic compounds. One should therefore expect that theextraction rate is limited by two consecutive mechanisms: Firstly, aleaching process controlled by a mass transfer at the solid/waterinterface, and secondly a mass transfer from aqueous to organic phasethrough the liquid/liquid interphase. One would assume both theseprocesses to be nearly proportional to the water solubility of thecompounds. Thus, the procedure according to the invention gives apositive and surprising effect of securing an almost complete leachingof almost water-insoluble compounds present in amounts even less than10% of other solid particles in an aqueous suspension.

We claim:
 1. A process for enriching and extracting compounds of lowwater solubility from an aqueous suspension consisting essentially ofsubstantially inorganic solid substances containing less than 10% byweight of said compounds of low water solubility, which comprisesacidifying said aqueous suspension, contacting said acidified suspensionwith an organic solvent, extracting said acidified suspension with saidorganic solvent in at least two mixing stages with subsequent phaseseparation after each mixing stage, at least the first of said mixingstages comprising at least two mixing sequences arranged in seriesupstream of a phase separation where two liquid phases are separatedfrom each other, a major proportion of said organic solvent resultingfrom said phase separation being recirculated to the first mixingsequence in the corresponding upstream mixing stage.
 2. A processaccording to claim 1, wherein the aqueous suspension is extracted withthe organic solvent in two or more continuous countercurrent stages. 3.A process according to claim 1 or 2, wherein the aqueous suspension isacidified to pH 3, whereafter the suspension is treated by continuousmixing and settling with a substantially recirculating organic solvent.4. A process according to claim 3, wherein the aqueous suspensioncontains up to 20% solid matter.
 5. A process according to claim 4,wherein the organic solvent is an aromatic petroleum fraction with aflashpoint above 25° C.
 6. A process according to claim 5 wherein theflashpoint is above 60° C.
 7. A process according to claim 3, whereinthe aqueous suspension is acidified to pH
 1. 8. A process according toclaim 2, wherein the ratio of organic to aqueous phase in the mixingsequences is greater than 1:2, rendering the organic solvent thecontinuous phase, thereby improving phase separation after each mixingstage.
 9. A process according to claim 8, wherein the ratio of organicto aqueous phase is greater than 1:1.
 10. A process according to claim1, wherein the compounds of low water solubility are chlorinatedhydrocarbons.